District-scale alteration associated with massive sulfide deposits in the Mount Read Volcanics, western Tasmania

Abstract

The distribution of secondary mineral assemblages in six mineralized areas of the Mount Read Volcanics defines a district-wide zonation which is subconformable with massive sulfide host horizons and the contacts of sill-like granitoids where present. The relationships between alteration patterns and elements of Cambrian geology strongly suggest that the alteration is related to Cambrian hydrothermal circulation responsible for massive sulfide mineralization. There is a later greenschist facies metamorphic overprint. The major components of the principal assemblages are: quartz + albite + chlorite + epidote; quartz + albite + chlorite + calcite; quartz + sericite + chlorite; and quartz + K-feldspar + chlorite. Several complex assemblages involving the above minerals; biotite, magnetite, pyrite, and tourmaline are characteristic of granite contacts. Secondary biotite is also present sporadically in the Rosebery area, probably related to Devonian granitoids at depth. There is a districtwide zonation of sodic + or - calcic versus nonsodic, noncalcic assemblages in felsic volcanic rocks. In preore volcanics, nonsodic, noncalcic assemblages occur immediately beneath the host horizon and may persist several hundred meters before passing into a zone, including granite contacts, where calcium minerals and albite occur sporadically. The nonsodic, noncalcic assemblages are predominantly sericitic, but assemblages with prominent K-feldspar occur locally. In postore volcanics, sericitic non-sodic, noncalcic assemblages may persist up to 200 m above the ore horizon before passing into relatively fresh volcanics with common albite and calcic greenschist minerals. Preore assemblages probably largely reflect downward circulation. Upwelling zones are characterized by intense sericite + chlorite or chlorite development with or without K-feldspar; lateral spreading of solutions immediately beneath the ore horizon is common.Chlorite compositions preserve local patterns due to Cambrian hydrothermal processes, but they do not vary systematically with alteration assemblages throughout the province. Chlorite Mg to Fe ratios appear to have been preserved through metamorphism. A combination of structural and alteration data is useful to mapping in the Mount Read Volcanics. This approach indicates that most syngenetic deposits and sedimentary lenses in the Rosebery-Hercules area may lie on a single host horizon and suggests a continuity of geology from Mount Farrell through Red Hills to Mount Sedgwick.